The performance of a rechargeable lithium ion battery (secondary battery or accumulator; these terms are used here as synonyms as is usual in the art, and hereinafter the term “battery” will often be used as an abbreviation) is measured as stored energy per mass (unit: mAhr/g) and depends on various factors.
It is a decisive parameter how much lithium can be intercalated into the two electrodes, anode and cathode, per gram of electrode mass. Other factors are the shelf life (lifetime of a non-loaded battery), the extent of self-discharge, the rate of recharging and the cycleability, i.e. no change in the charging capacity between repeated charging and discharging cycles should occur if possible. Last but not least, the manufacturing cost and consequently the price-performance ratio are to be kept within the scope of marketability.
Today's commercially available lithium ion batteries have to be still improved considerably in terms of their performance and price-performance ratio before a large-scale use in the field of energy becomes possible. Huge rising markets which generally require a considerably improved energy storage technology are e.g. accumulators for the “electric car” or for energy storage associated with alternative power generation.
WO 2007/027197 A2 deals with the problem to improve the cycleability of lithium ion batteries and proposes for this purpose a cathode comprising nanowires made of a lithium cobalt oxide, which stand upright on a metal film (formed of titanium and platinum here). The nanowires are formed by electrodeposition in pores of a porous aluminum oxide layer which is deposited especially as a template on the metal film. The template is then completely removed by etching with NaOH or KOH. On the metal film, a plurality of isolated upright nanowires (also referred to as “nanorods”) remains, which have defined diameters and distances from each other. In other words, a nanorod array has been formed on the metal film.
The method of WO 2007/027197 A2 includes a number of depositing and patterning steps following the formation of the metal film, making the method very time-consuming and expensive. The nanowires are produced during the process and adhere directly on the metal film with one of their ends.
For the anode of a lithium ion battery, the paper by Chan et al. (“High-performance lithium battery anodes using silicon nanowires” Nature Nanotechnology 3, 31 (2008)) proposes that silicon nanowires should be arranged in an upright manner on a metal film (charge collector). It has been known for a long time that silicon, under formation of silicon-lithium compounds, can intercalate about 11 times more lithium per gram of silicon than a technically common graphite electrode. The capacitance of this anode exceeds 4000 mAhr/g which is even larger than that of metallic lithium. However, previous experiments for the use of silicon anodes have failed, because practically no cycleability was achieved. The reason for the extremely poor cycleability of silicon is its volume expansion involved in the intercalation of lithium by a factor of 4. The stresses occurring in this case are so large that the material is really powdered (degraded).
The paper by Chan et al indicates the fundamental solution to the problem: silicon nanowires are grown on e.g. a steel substrate by well-known techniques (here: liquid-vapor-solid method; LVS). The nanowires are flexible and can double their diameter without breaking. Nanopatterning of silicon increases the surface for receiving lithium ions, on the one hand, and makes place for preventing the above-mentioned stresses, on the other.
However, the anode of Chan et al is not yet ready for large-scale production. This is, above all, due to the manufacturing process which is complicated and expensive. The growth of the silicon nanowires by LVS methods requires gold particles as nucleation sites which remain on the tips of the nanowires. The nanowires themselves are saturated with gold, making the production of thicker wires or on larger areas very costly. In addition, the nanowires obtained are not homogeneous. There are thick and thin, long and short, upright and bent nanowires and nanowires securely attached to the substrate and nanowires detaching from it.
Silicon nanowires which are not contacted with the metal film are particularly undesirable in large-scale manufacture. They do not contribute anything to the ampere-hour capacity of the battery, but when the battery is charged for the first time, they absorb lithium ions which cannot be extracted (irreversible capacity). And of course, they also are saturated with gold in the method by Chan et al.
In particular the cycleability of the silicon-nanowire anode according to Chan et al is not too good as recent results reveal (ECS Conference, San Francisco, May 2009). The electrical connection degrades with the number of charging and discharging cycles. The volume expansion of silicon seems to result at least in stresses in the area of the contact point on the metal film where the silicon nanowires adhere. Chan et al write the following about their nanowires: “They also appeared to remain in contact with the current collector, suggesting minimal capacity fade due to electrically disconnected material during cycling.” (Nature Nanotechnology 3, 31 (2008)). This implies that nanowires detach during the cycles at least in a small number. However, an optimum electrical connection requires that each nanowire is connected to, and above all is kept connected to, the current-collecting electrode at a low resistance.
Silicon-nanorod arrays are known in the state of the art also from a completely different technical field, that is, electrochemical pore etching in semiconductor wafers.
It is, for example, apparent from the paper by van Katz et al (“Synthesis of Monodisperse High-Aspect-Ratio Colloidal Silicon and Silica Rods”, Langmuir 2004, 20, 11201-11207), that the pore diameter of p-type silicon can be varied by changing the etching current. From given current intensities, the pores begin to overlap (overetching the pores), i.e. the pore space generated is connected throughout the etched region of the wafer. Only remains of the original pore walls are left and these protrude from the wafer as isolated rods. Van Katz et al then break off the nanorods by rinsing with water or ethanol.
Noteworthy here is the contrast to the nanorods from the paper by Chan et al: Due to the homogeneity of the etching process, all rods are practically the same in terms of length, diameter and even cross-sectional shape. They are made of pure silicon (without any additions such as gold or the like) and are also identical in terms of their material structure, as they are formed from the same wafer.
However, the silicon wafer provided with nanorods by etching is not suited as an anode of a lithium ion battery, as lithium ions would also penetrate into the wafer and would degrade it, which would result in the detachment of all nanowires. In addition, silicon itself is not a good conductor and hence the wafer is not a good charge collector.